The present invention relates to a process for producing a high quality carbon fiber. More specifically the invention relates to a rapid oxidation step for improving the efficiency and economics of carbon fiber production from PAN-fibers. A herein disclosed improved PAN-fiber allows for swift oxidation while minimizing temperature surges within the fiber and spreading heat release over a longer time.
Carbon fibers prepared from acrylonitrile polymers and copolymers by a rapid oxidation process have superior physical properties such as increased tensile strength. The fibers are useful as reinforcement materials in automobile, aerospace, recreational and various other industries. An increasing demand for strong lightweight materials insures an expanded use of carbon fibers in the future. Thus a need exists for a process which insures that the starting materials for producing carbon fibers are of the finest quality. A fine quality acrylonitrile polymer or copolymer has no defects such as voids formed when gases are expelled during fiber preparation. Also the fiber should not contain more than traces cf metal contaminants, as these tend to degrade the fiber. The fiber should have a round shape for maximum stiffness.
Carbon fibers, which have heretofore been used as reinforcing material for plastic composite compositions, are preferably characterized by high tensile strength, high rigidity and a homogeneous fibrous structure. These characteristics can be adversely affected by certain properties found in the acrylonitrile copolymer feedstocks. If these undesirable properties can be identified and removed, then the final carbon fiber product is greatly enhanced in desirable characteristics.
Polyacrylonitrile (PAN)-based carbon fibers are produced in a process comprising three steps. A relatively low temperature heat treatment or oxidation step is followed by a carbonization step. The third step is an optional high temperature heat treatment. During the first step of oxidative heat treatment, a well-oriented ladder polymer structure is developed under tension.
The oxidation step is critical to the development of a high strength carbon fiber material. Prior to this step, the PAN-fibers are frequently stretched by 100% to 500% at a temperature of about 100.degree. C. The stretching improves the alignment in the polymer structure and reduces the fiber diameter, as well as increasing the tensile strength and Young's modulus of the final carbon fiber.
In the past, the oxidation step has been conducted for a time of about 1 to about 5 hours. The step is slow and adds significant expense to the overall process. Process temperatures must be maintained below the fusion temperature of the fibers to prevent instantaneous temperature surges within the fiber. Temperature surges produce bubbles of gaseous products which ruin the physical properties of the carbon fiber. The oxidation step is conducted in an oxidizing atmosphere, usually air, at a temperature of about 190.degree. C. to about 280.degree. C. The reaction is an exothermic one, and a runaway reaction is always possible.
The carbonization step which follows the oxidation step is performed rapidly in an inert atmosphere at a temperature of about 1000.degree. C. to about 2000.degree. C. Tensile strength of the fiber reaches a maximum in this step.
U.S. Pat. No. 5,462,799 discloses the preparation of a carbon fiber wherein a precursor PAN-fiber is oxidized, carbonized and if necessary graphitized to make the carbon fiber of specified surface oxygen concentration, specified surface concentration of hydroxyl groups and specified surface concentration of carboxyl groups.
U.S. Pat. No. 5,281,477 discloses the preparation of a carbon fiber having high tenacity and high modulus of elasticity. Pretreated fibers are passed through a first carbonization zone, a second carbonization zone and a third carbonization zone.